The estrogen receptor antagonist tamoxifen opposes estrogen driven cell proliferation in the breast and consequently is widely used as adjuvant hormonal therapy for breast cancer. However, in the endometrium of a large proportion of treated patients it manifests partial ER agonist activity. In view of the growing interest in developing tamoxifen (or a related compound) as a prophylactic agent for breast cancer, it is imperative that the molecular events regulated by tamoxifen in this tissue be defined in detail. Thus, one major objective of our proposal is to identify the gene products expressed in the endometrium in response to tamoxifen and to define specifically which of these genes are responsible for uterine cell proliferation. Recently, our laboratory has shown that the mechanism of action of tamoxifen and the structurally related antiestrogen raloxifene are distinct. Additionally, our data suggests that raloxifene may have additional benefits over tamoxifen as a chemopreventative for breast cancer. Since it is unlikely that many compounds will be evaluated clinically as chemopreventatives, we propose additionally to identity the specific gene products induce by raloxifene in the human uterus. It is anticipated that the identified genes will serve as specific markers of antihormone action in the endometrium permitting the assessment of their estrogenic activity in advance of observable histological changes. To accomplish this objective we have developed a combined clinical and basic science approach to identify the gene products regulated in the human endometrium following treatment with tamoxifen and raloxifene. In this grant we propose to collect serial endometrial biopsies from postmenopausal women treated with tamoxifen or raloxifene and use this tissue to (l) observe and record the histological changes in the endometrium of women on tamoxifen or raloxifene, (2) identify the specific gene products regulated by these antiestrogens and (3) test the significance of the distinct gene expression patterns with observed histological alterations in the uterus. To facilitate the identification of antihormone regulated genes we have developed and validated a genetic system in yeast to clone tamoxifen and raloxifene responsive promoters from genomic DNA from endometrial cell nuclei. Since this system is based on the genomic copy number of a given gene, rather than its absolute level of expression, it should allow the identification of genes which are important but which are expressed at low levels in target cells. This unique approach, coupled with the clinical studies proposed, will serve to develop a series of molecular criteria with which to evaluate the chemotherapeutic potential of estrogen receptor modulators in advance of establishing large and costly chemoprevention trials.